The capacity of a rotary screw compressor in refrigeration and heating pump systems is usually controlled by one or more axially displaceable sliding valves, for instance of the SRM type, see FIG. 1 of the accompanying drawings. If a controllable slide stop is added, the built-in volume can also be varied.
A regulating principle which is used less seldom is one in which lift valves regulate the capacity of the rotary screw compressor and vary the built-in volume. These valves operate instantaneously and are either open or closed. When they close towards the operating chamber, the valves are subjected to the pressure in the operating chamber which, due to pulsations, pressure drops, and so on, may be higher than the outlet pressure of the compressor, and is higher than this pressure at the outlet. The inwardly directed pressure on the valves must therefore be greater than that directed outwardly from the operating chamber, in order to ensure that the valves are closed and to prevent them operating unsteadily (chattering).
A simple construction of the lift valve is particularly attractive in the case of small compressors which, for commercial reasons must be inexpensive and simple in function. These compressors thus often lack an oil pump or other means of increasing pressure and the outlet pressure of the compressor is the highest pressure in the system, even outside the operating chamber of the compressor. It is thus this highest system pressure which is to control the movement of the lift valves.